A note on the limit loads of non-standard materials

Summary The paper discusses the collapse theorems for materials which do not exhibit normality of the strain rate vector to the yield surface in the superposed stress and strain rate spaces. The consequences of the lack of normality on the designs for minimum weight are considered.

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Sommario Si presentano i teoremi fondamentali dell'analisi limite con riferimento a materiali per i quali i vettorï velocità di deformazione plastica non sono normali alla superficie rappresentativa, nello spazio delle tensioni, della condizione di plasticità. Se ne considerano quindi le conseguenze sui progetti di minimo peso.

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Study supported by the C.N.R. (Plasticity Group).

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We use here the term non-standard materials as currently used in the French literature. English-speaking Authors usually prefer the equivalent expression materials with non-associated flow law.     

Limit analysis and design of structures having elements with random distribution of yield stresses,with associated and non-associated flow laws

Summary A generic structure is considered, made up of elements with rigid perfectly plastic standard material and random plasticity constant, however distributed around the average value. The limit load of structures having elements of average plasticity constant is shown to be greater than or equal to the average value of the limit loads of all the feasible structures, thus generalising a theorem [1] formulated for framed structures.The minimum volume of structures having elements of average plasticity constant is then shown to be less than or equal to the average value of the minimum volumes of all the feasible structures. Then the earlier theorems are extended to take in non standard materials.

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Sommario Si considera una generica struttura composta da elementi di materiale rigido-plastico standard a costante di plasticità aleatoria e comunque distribuita attorno al valor medio.Si dimostra che il carico di collasso della struttura dotata di elementi a costante di plasticità media è maggiore o uguale al valor medio dei carichi limite di tutte le strutture possibili, generalizzando un teorema [1] formulato per le travature reticolari. Si dimostra poi che il minimo volume della struttura dotata di elementi a costante di plasticità media è minore o uguale al valor medio dei minimi volumi di tutte le strutture possibili. Si estendono quindi i teoremi precedenti al caso di materiali non standard.

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This work has been sponsored by C.N.R. (the Italian Council of Research).     

Chemical changes induced by sputtering in TiO2 and some selected titanates as observed by X-ray absorption spectroscopy

We studied the chemical changes induced by Ar⁺-bombardment in TiO₂ and some selected titanates by means of soft X-ray absorption spectroscopy. The O 1s and Ti 2p spectra provide unique information on hybridization, valency and crystal-field effects. In TiO₂, we found that some Ti ions are partly reduced but no metallization is observed. On the other hand, the effect of ion-bombardment in titanates depends on the counter-ions. In Al₂TiO₅, the central Ti ions are strongly reduced while Al₂O₃ remains unaffected. In NiTiO₃, by contrast, we found the opposite behaviour with strong reduction of the counter Ni ions. This contrasting behaviour seems to be related to differences in the relative heat of formation of the sub-species. In PbTiO₃, both central- and counter-ions are affected by sputtering. Finally, the electronic structure of BaTiO₃ presents very little changes upon ion-bombardment.     

Sol-gel transition in worm-like micelles

Summary The statical and dynamical properties of lecithin/H₂O/cyclohexane cylindrical reverse micelles are investigated as a function of lecithin volume fraction, ϕ, and temperature at fixed water/lecithin ratio,w ₀. The viscosity data are well fitted by the Cates model when the breathing mode of micelles is taken into consideration, overlapping with the breaking and reforming mechanisms. We present some results from Brillouin-scattering experiment, performed across the sol-gel transition. In order to explain the experimentally observed ϕ-dependence of the hyperacoustic parameters, a mechanical model was developed from which the ϕ-dependence of the micelle size distribution was obtained. From a comparison with the viscosity data the entanglement length was estimated. Furthermore some new results from an incoherent quasi-elastic neutron scattering experiment are presented. The whole body of the experimental results suggests for a sol-gel transition triggered by topologic phenomena. When the lecithin volume fraction increases, the kinetic equilibrium between the breaking and reforming mechanisms of the micelles shifts the mean micelle length towards higher values and the entanglement of the micelles becomes highly favourable. The obtained results are discussed and compared with other findings in literature. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Intramolecular cyclization of 4-acetyl-2,2-diethoxycarbonyl-2,3-dihydro-1-(2-nitrobenzoyl)pyrrole by catalytic reduction

Catalytic hydrogenation of 4-acetyl-2,2-dicthoxycarbonyl-2,3-dihydro-1-(2-nitrobenzoyl)pyrrole 2 afforded 3-acetyl-1,1-diethoxycarbonyl-1,2,3,3a,4,9-hexahydro-9-oxopyrrolo[2,1-b]quinazoline 3 and the 4-hydroxy derivative 4 , together with two tetrahydro analogs 5 and 6 of 3 . Compounds 5 and 6 were also obtained by dehydration of 4 .     

A study of some parameters relevant to the response of harwell PMMA dosimeters to gamma and electron irradiation

The effects on the radiation response of Harwell polymethylmethacrylate (PMMA) dosimeters of dose-rate, radiation type, temperature during irradiation and post-irradiation storage, and post-irradiation stability, are of importance to the operators of commercial irradiation facilities.

This paper describes recent studies of the effects of some of these parameters on the radiation response of Harwell Red 4034, Amber 3042, and Gammachrome YR Perspex dosimeters, and provides data on batch to batch variation and shelf-life.     

Detection of Copper(II) in Water by Methylene Blue Derivatives

Two novel phenothiazin-5-ium derivatives bearing cyclam moieties ( 3 a⁺ and 3 b⁺ ) were synthesized and investigated as Cu²⁺ sensors. Both ligands show intense spectral changes in the presence of Cu²⁺ in aqueous solutions. The high molar extinction coefficient of the chromophore allows both naked eye detection and spectrophotometric quantitative determination of the cation at a micromolar-concentration scale. 3 a⁺ was found to outperform 3 b⁺ , showing negligible spectral changes in the presence of excesses of other metal cations.